Measure of quality of link between mobile station and base station

ABSTRACT

A mobile station associated with a base station determines a measure of the quality of a communication link between the mobile station and the base station. The measure is determined by comparing either i) an expected data rate for communications over the link and an actual data rate of communications over the link, wherein the expected data rate takes into account radio frequency (RF) power levels of signals received at the mobile station over the link, or ii) an expected RF power level for communications over the link and RF power levels of signals received at the mobile station over the link, wherein the expected RF power level takes into account the actual data rate of communications over the link. This measure of the quality of the link may be a factor in triggering the mobile station to initiate a handoff.

BACKGROUND

As used herein, the term “handoff” is intended to include transfersbetween networks of the same type and networks of different types, andalso to include transfers while a mobile station is in idle mode andtransfers while a mobile station is in an active communication sessionsuch as a telephone call.

A mobile station that is able to communicate with access points (APs) ofwireless local area networks (WLANs) will typically make a decision totrigger a handoff based on the received signal strength indicator (RSSI)of signals received at the mobile station. The decision may be based onsignals that originate from the AP with which the mobile station iscurrently associated or on signals that originate from other APs or onboth types of signals. The other APs may belong to the same WLAN as theAP with which the mobile station is currently associated or may belongto a different WLAN. Handoffs from one WLAN to another may require moretime to complete than handoffs within the same WLAN. For example, if themobile station needs to obtain a new Internet Protocol (IP) address aspart of the handoff, then the handoff will typically require more timethan if the mobile station could maintain its current IP address. New IPaddresses may be required if the mobile station performs the handoffacross networks or across subnets of the same network.

If the mobile station is also able to communicate with base stations ofa wireless wide area network (WWAN), for example, a cellular telephonenetwork, then the decision to trigger a handoff from a WLAN to thecellular network is also typically made based on the RSSI of signalsreceived at the mobile station. The decision may be based on signalsthat originate from the AP with which the mobile station is currentlyassociated or on signals that originate from one or more of the basestations or on both types of signals. Handoffs from a WLAN to a cellularnetwork typically require more time to complete than handoffs from oneAP to another AP.

While many different schemes for triggering handoffs have been proposed,most schemes are directed at achieving one or more of the followinggeneral objectives:

a) A mobile station should look to perform a handoff before it loses itsWLAN connection;

b) A handoff ought to take a minimum amount of time; and

c) A handoff from a WLAN to a cellular network ought to be completedwhile the mobile station is still in an area of overlapping coveragebetween the WLAN and the cellular network.

However, it is well known that under certain circumstances the handoffsfail to achieve these objectives. If a WLAN connection is lost or if itslink quality deteriorates significantly before or during a handoff,communications over the WLAN connection may suffer. For example, atelephone call being carried over the WLAN connection may bedisconnected or suffer unacceptable noise or delays.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example and not limitation in thefigures of the accompanying drawings, in which like reference numeralsindicate corresponding, analogous or similar elements, and in which:

FIG. 1 is a schematic illustration of an exemplary communication system;

FIG. 2 is a block diagram of an exemplary mobile station;

FIG. 3 is a flowchart of an exemplary method for calculating a measureof the quality of a link between a mobile station and a base station;

FIG. 4 is a flowchart of an exemplary method for determining a measureof the quality of a link based on the results of a comparison between anexpected data rate and the actual data rate;

FIG. 5 is a flowchart of another exemplary method for calculating ameasure of the quality of a link between a mobile station and a basestation; and

FIG. 6 is a flowchart of an exemplary method for determining a measureof the quality of a link based on the results of a comparison betweenthe measured RF power level and an expected RF power level as derivedfrom the actual data rate.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of embodiments.However it will be understood by those of ordinary skill in the art thatthe embodiments may be practiced without these specific details. Inother instances, well-known methods, procedures, components and circuitshave not been described in detail so as not to obscure the embodiments.

Some wireless local area networks (WLANs) operate in an unlicensedfrequency band and may therefore be subject to interference fromnon-WLAN devices operating in the same frequency band. For example, IEEE802.11 WLANs, also known as “Wi-Fi networks”, operate in the unlicensed2.4 GHz and 5 GHz bands. The 2.4 GHz band is also used by cordlesstelephones, microwave ovens, baby monitors, Bluetooth® devices, ZigBee™devices and WirelessUSB™ devices. The lower portion of the 5 GHz band isused by satellite to ground links, and the middle portion of the 5 GHzband is used by RADAR devices. There is a general assumption amongpersons of ordinary skill in the art that since the cellular spectrum islicensed, it is relatively clean of interference.

WLAN devices have multiple data rates at which they can communicate. Forexample, IEEE 802.11b devices can communicate at 1 Mbps (megabits persecond), 2 Mbps, 5.5 Mbps and 11 Mbps. IEEE 802.11g/a devices cancommunicate at 1 Mbps, 2 Mbps, 5.5 Mbps, 6 Mbps, 9 Mbps, 11 Mbps, 12Mbps, 18 Mbps, 24 Mbps, 36 Mbps, 48 Mbps, and 54 Mbps. As is well knownin the art, if a signal is received at a certain power level, one canexpect a certain data rate for the signal in keeping with thesignal-to-noise ratio (SNR).

In the presence of interference, various error rates or loss rates orboth may increase. For example, any of the bit error rate (BER), thepacket error rate (PER), and the packet loss rate (PLR) may be such thatthe WLAN devices are forced to communicate at lower data rates, eventhough the radio frequency (RF) transmission power level of the devices(and hence, the received signal strength) remains high. There are alsoother situations in which received signal strength may be high, yetcommunication will be conducted at a lower data rate than what isexpected for that received signal strength. For example, certain APs mayhave been configured to communicate at lower data rates than what acommunication standard allows.

As explained in the Background, typical mobile stations are triggered toroam based on the RSSI of signals received at the mobile station. If theRSSI is high, a typical mobile station communicating at a relatively lowdata rate with an AP will not be prompted to seek an AP with which itcan communicate at a higher data rate. Likewise, if the RSSI is high, atypical mobile station that is also capable of cellular communicationswill not be prompted to seek a cellular base station with which it cancommunicate at a higher data rate. Likewise, if the RSSI is high, atypical mobile station that is also capable of Wireless MetropolitanArea Network (WMAN) communications, for example, WorldwideInteroperability for Microwave Access (WiMAX) communications, will notbe prompted to seek a Head End (WMAN) or base station (WiMAX) with whichit can communicate at a higher data rate.

In the following description and claims, the term “base station” isintended to encompass an access point (AP) of a WLAN (e.g. a Wi-Finetwork), a base station of a WWAN (e.g. a cellular network), and a HeadEnd of a WMAN (e.g. a WiMAX network).

In one aspect, a mobile station associated or otherwise connected with abase station determines a measure of the quality of a communication linkbetween the mobile station and the base station. The measure takes intoaccount (a) RF power levels of signals received at the mobile stationover the link and (b) an actual data rate of communications over thelink. The actual data rate may have been derived by the mobile stationand the base station, or may be the result of negotiation or the use ofa data rate adaptation algorithm or both. For example, a data rateadaptation algorithm may drop or lower the data rate in response to twopacket errors occurring in a short time period, and may raise orincrease the data rate in response to having three or more successfulpacket transmissions occurring in a similar or longer time period. Thismeasure of the quality of the link may be a factor in triggering themobile station to initiate a handoff.

In some cases, the mobile station may preferentially select as itshandoff target a base station known to have less interference. Forexample, if a base station operating in the 5 GHz band is available, themobile station may prefer to roam to that base station rather than tobase stations operating in the 2.4 GHz band.

Determining the measure of the quality of the link may involvedetermining an expected data rate for communications over the link basedon the power levels, and comparing the expected data rate to the actualdata rate. If the actual data rate is lower than expected for themeasured RF power level, then it may be an indication that the linkquality is suffering from degradations. The degradation may be due tointerference or multipath or other causes of degradation or anycombination thereof. As the degradation increases, the measure of linkquality will decrease. The degree to which the actual data rate is lowerthan expected for the measured RF power level may therefore be used as ameasure of the degradation. The mobile station may use the measure oflink quality to infer the presence of degradations on the link.

The measure of link quality may be abstracted to indicate whether theactual data rate is at or lower than an expected data rate for themeasured RF power level. For example, the measure of link quality may beabstracted as high, medium or low. If the actual data rate is asexpected or is close to as expected for the measured RF power level,then the measure of link quality may be considered high. If the actualdata rate is lower than expected for the measured RF power level, thenthe measure of link quality may be considered low or medium, dependingon how much lower the actual data rate is than the expected data rate.

RSSI is a measure of the RF power level, but is known to vary with thenoise level. Other measures of the RF power level are also contemplatedin the determination of the measure of link quality. For example, thereceived channel power indicator (RCPI) defined in IEEE 802.11k isintended to measure the RF power in the presence of noise and may beused to determine the measure of the quality of a communication link.Measurements used to determine the RF power level may be averaged overan interval, the duration of which may be configurable. For example, theRSSI or RCPI measurements may be averaged over a number of packetscommunicated over the link.

A mobile station may be associated or otherwise connected with more thanone base station at the same time. For example, the mobile station maybe capable of communications with different types of networks and may beassociated or otherwise connected with different base stations ofdifferent types of networks. A measure of the quality of a communicationlink may be determined for any of the links between the mobile stationand the different base stations. For each link, the measure takes intoaccount (a) RF power levels of signals received at the mobile stationover the link and (b) an actual data rate of communications over thelink.

The link quality measures of more than one communication link may befactors in determining whether to trigger the mobile station to initiatea handoff. Similarly, a target for the handoff may be selected fromamong the different base stations based, at least in part, on themeasure of the quality of the link between the mobile station and thetargeted base station. For example, if a mobile station issimultaneously connected to a WLAN base station and to a WiMAX basestation, and the mobile station is handling a communication session viathe WiMAX base station, the mobile station may determine to transfer thecommunication session to the link between the mobile station and theWLAN base station. This determination may be based, at least in part, onthe measures of the quality of the links between the mobile station andWiMAX base station and WLAN base station, respectively.

FIG. 1 illustrates an exemplary communication system 100. Communicationsystem 100 comprises several APs, of which APs 102, 104, 106 and 108 areshown. Any of APs may provide a connection to a general access network(GAN) controller, to a Call Session Control Functions (CSCF) server, orto any other suitable type of infrastructure. As a mobile station 110 iscarried along a trajectory 111 through the coverage areas 112, 114, 116and 118, respectively, of APs 102, 104, 106 and 108, mobile station 110may calculate a measure of the quality of a wireless link between itselfand the AP with which it is currently associated. The link qualitymeasure, which is a function of the actual data rate and the RF powerlevel of communications on the link, may be a factor in triggeringmobile station 110 to perform a handoff from the AP with which it isassociated.

At a location 120, for example, mobile station 110 may be associatedwith AP 102. Mobile station 110 may calculate a measure of the qualityof a wireless link 122 with AP 102, where the measure takes into accountthe strength of signals from AP 102 received at mobile station 110 andthe data rate which mobile station 110 and AP 102 have derived forcommunications over wireless link 122. Mobile station 110 may thendetermine whether to trigger initiation of a handoff from AP 102 based,at least in part, on the link quality measure it has calculated. Otherfactors may also be considered by mobile station 110 when determiningwhether to trigger initiation of a handoff. Once triggered, mobilestation 110 may scan for other APs, detect AP 104 and perform a handofffrom AP 102 to AP 104. AP 102 and AP 104 may belong to the same subnetof a WLAN.

At a location 124, mobile station 110 may trigger initiation of ahandoff from AP 104, scan for other APs, detect AP 106 and perform ahandoff from AP 104 to AP 106. AP 104 and AP 106 may belong to differentsubnets of the same WLAN, and therefore the handoff from AP 104 to AP106 may take longer to complete than the handoff from AP 102 to AP 104.

At a location 126, mobile station 110 may trigger initiation of ahandoff from AP 106, scan for other APs, detect AP 108 and perform ahandoff from AP 106 to 108. AP 106 and AP 108 may belong to differentWLANs, and therefore the handoff from AP 106 to AP 108 may take longerto complete than the handoff from AP 102 to AP 104.

At a location 128, mobile station 110 may trigger initiation of ahandoff from AP 108, scan for other APs and not detect any other APs.The WLAN connection may be lost as mobile station 110 exits the coveragearea 118 of AP 108.

If, however, mobile station 110 is also able to communicate with basestations of cellular networks, then at location 128, mobile station 110may also search for base stations of cellular networks with which it iscompatible and authorized to establish a service connection. If, forexample, at location 128, mobile station 110 detects a base station 130of a cellular network, mobile station 110 may perform a handoff from AP108 to base station 130. Likewise, assuming that the coverage area ofbase station 130 encompasses the coverage areas of APs 102-108, mobilestation 110 may have performed a handoff to base station 130 at any oflocations 120, 124 and 126 to the cellular network.

Alternatively, mobile station 110 may have been connected with basestation 130 while connected with AP 102 and when measuring the qualityof link 122 while at location 120, mobile station 110 may have alsomeasured the quality of a link between mobile station 110 and basestation 130. Based on the two link quality measures, mobile station 110may have determined to transfer a communication session being handledvia AP 102 to the link between mobile station 110 and base station 130.

FIG. 2 is a block diagram of an exemplary mobile station. A mobilestation 200 comprises a processor 202, and a memory 204 coupled toprocessor 202. Memory 204 stores the components of link qualitymeasurements and data rate in a module 206 that, when executed byprocessor 202, may implement the methods described herein.

Mobile station 200 comprises a wireless communication interface 210compatible with a first wireless communication standard. For example,wireless communication interface 210 is compatible with one or more WLANstandards, for example, one or more standards of the family of IEEE802.11 wireless communication standards. Wireless communicationinterface 210 is coupled to processor 202 and includes at least a WLANcontroller 212 and a radio 214. Mobile station 200 also comprises anantenna 216 coupled to radio 214. For example, mobile station 200 may beable to communicate with APs via wireless communication interface 210and antenna 216.

Memory 204 also stores a handoff application module 208 that, whenexecuted by processor 202, determines when to initiate handoff from anaccess point and controls wireless communication interface 210accordingly.

Mobile station 200 may also comprise a wireless communication interface220 compatible with a second wireless communication standard. Forexample, wireless communication interface 220 is compatible with one ormore wireless WWAN/WMAN communication standards. Wireless communicationinterface 220 is coupled to processor 202 and includes at least abaseband controller 222 and a radio 224. Radio 224 may be coupled toantenna 216, or mobile station 200 may comprise an additional antenna226 coupled to radio 224. Mobile station 200 may be able to communicatevia wireless communication interface 220 and antenna 216 or 226 withbase stations of a WWAN/WMAN network.

Mobile station 200 includes other components that, for clarity, are notshown in FIG. 2. A non-exhaustive product list of examples for mobilestation 200 includes a wireless-enabled laptop computer, awireless-enabled tablet computer, a wireless-enabled cellphone, awireless-enabled personal digital assistant (PDA), a wireless-enabledsmart phone, a wireless-enabled video camera/monitor, a wireless-enabledgaming/multimedia console, a wireless-enabled sensor/reporting/storagedevice, a wireless Internet Protocol (IP) phone and any other suitablemobile station.

FIG. 3 is a flowchart of an exemplary method for calculating a measureof the quality of a link between a mobile station and a base station. At302, the mobile station measures the RF power level of signals receivedat the mobile station from the base station with which the mobilestation is currently associated or otherwise connected. The RF powerlevel may be measured, for example, in dBm, which is decibels relativeto a power level of one milliwatt. For example, the mobile stationobtains signal strength measurements of the signals. Signal strengthmeasurements, for example, RSSI, may be averaged over a time interval,the duration of which may be configurable. The duration of the intervalmay be a number of packets, with an increasing number of packetsimproving measurement resolution/accuracy and reducing measurementerrors with the tradeoff or cost of overall measurement time delay torespond to a valid condition. In another example, the mobile stationobtains RCPI measurements of the signals, which may be averaged over aninterval, the duration of which may be configurable.

At 304, link quality measurement module 206 may determine an expecteddata rate based on the RF power level measured at 302. A look-up tablemay be employed to determine the expected data rate. The look-up tablemay be based on field measured data, laboratory measured data, expectedreceiver performance, manufacturing calibration data, or any combinationthereof.

An example of such a look-up table for IEEE 802.11b devices follows,assuming a data rate algorithm derives a packet error rate of <1 in 1000and the radio noise floor plus other radio implementation errors equal−95 dBm:

Received Power Level Expected Data Rate −83 dBm or higher 11 Mbps  −86dBm to −84 dBm 5.5 Mbps   −89 dBm to −87 dBm 2 Mbps −90 dBm or lower 1Mbps

An example of such a look-up table for IEEE 802.11g/a devices follows,assuming a data rate algorithm derives a packet error rate of <1 in 1000and the radio noise floor plus other radio implementation errors equal−95 dBm:

Received Power Level Expected Data Rate −68 dBm or higher 54 Mbps −72dBm to −69 dBm 48 Mbps −76 dBm to −73 dBm 36 Mbps −80 dBm to −77 dBm 24Mbps −83 dBm to −81 dBm 18 Mbps −85 dBm to −84 dBm 12 Mbps −87 dBm to−86 dBm  9 Mbps −88 dBm or lower  6 Mbps

IEEE 802.11n, which is a proposed amendment to the IEEE 802.11 physical(PHY) layer, is expected to have many more data rates than IEEE 802.11bor IEEE 802.11a/g. This increase in the number of data rates means thatthe dBm ranges of received power level for an expected data rate will besmaller for any one rate.

Cellular technology currently has limited data rate adaptation, but itis contemplated that more data rate choices will be available in futurecellular network standards.

As is known in the art, a mobile station and the base station with whichit is associated or otherwise connected will derive or negotiate, oradapt, or any combination thereof, to an actual data rate forcommunications over the link between the mobile station and the basestation. At 306, link quality measurement module 206 may compare thisactual data rate to the expected data rate determined at 304.

At 308, link quality measurement module 206 may determine a measure ofthe quality of the link based on the results of the comparison done at304. The measure of the quality of the link may be abstracted to threevalues (high, medium, low). Abstractions to two values and abstractionsto more than three values are also contemplated.

At 310, link quality measurement module 206 may provide the measure ofthe quality of the link to handoff application module 208.

FIG. 4 is a flowchart of an exemplary method for determining a measureof the quality of a link based on the results of a comparison between anexpected data rate and an actual data rate. In this exemplary method,the quality of the link has been abstracted to three values: high,medium and low.

If the actual data rate is within x “steps” of the expected data rate onthe scale of allowable data rates, as checked at 402, then the qualityof the link is determined at 404 to be “high”. If the actual data rateis y or more “steps” below the expected data rate on the scale ofallowable data rates, as checked at 406, then the quality of the link isdetermined at 408 to be “low”. If neither of these conditions is met,then the quality of the link is determined at 410 to be “medium”. Thevalues of x and y will depend upon the total number of allowable datarates and on how the abstraction is to be handled.

For example, for IEEE 802.11b devices, x may be set to zero, and y maybe set to two. If the expected data rate (based on the RF power levelsof the received signals) is 11 Mbps and the actual data rate is 2 Mbpsor 1 Mbps, then the quality of the link is “low”.

In another example, for IEEE 802.11g/a devices, x may be set to one, andy may be set to three. If the expected data rate (based on the RF powerlevels of the received signals) is 48 Mbps and the actual data rate is18 Mbps or less, then the quality of the link is “low”. If the actualdata rate is 48 Mbps or 36 Mbps, then the quality of the link is “high”.If the actual data rate is 24 Mbps, then the quality of the link is“medium”.

It is obvious to a person of ordinary skill in the art how to modify themethod of FIG. 4 to abstractions of two values and to abstractions ofmore than three values.

It is also obvious to a person of ordinary skill in the art to modifythe methods described herein as follows: instead of determining anexpected data rate based on the RF power level and comparing the actualdata rate to the expected data rate, the method could involvedetermining an expected RF power level based on the actual data rate andcomparing the expected RF power level to a measured RF power level. Ifthe measured RF power level is higher than expected for the actual datarate, then it may be an indication that the link quality is sufferingfrom degradations. The degradation may be due to interference ormultipath or other causes of degradation or any combination thereof. Asthe degradation increases, the measure of link quality will decrease.The degree to which the measured RF power level is higher than expectedfor the actual data rate may therefore be used as a measure of thedegradation. The mobile station may use the measure of link quality toinfer the presence of degradations on the link.

The measure of link quality may be abstracted to indicate whether themeasured RF power level is at or higher than an expected RF power levelfor the measured actual data rate. For example, the measure of linkquality may be abstracted as high, medium or low. If the measured RFpower level is as expected or is close to as expected for the measuredactual data rate, then the measure of link quality may be consideredhigh. If the measured RF power level is higher than expected for themeasured actual data rate, then the measure of link quality may beconsidered low or medium, depending on how much higher the measured RFpower level is than the expected RF power level.

FIG. 5 is a flowchart of an exemplary method for calculating a measureof the quality of a link between a mobile station and a base station. At502, the mobile station measures the RF power level of signals receivedat the mobile station from the base station with which the mobilestation is currently associated or otherwise connected. The RF powerlevel may be measured, for example, in dBm, which is decibels relativeto a power level of one milliwatt. For example, the mobile stationobtains signal strength measurements of the signals. Signal strengthmeasurements, for example, RSSI, may be averaged over a time interval,the duration of which may be configurable. The duration of the intervalmay be a number of packets, with an increasing number of packetsimproving measurement resolution/accuracy and reducing measurementerrors with the tradeoff or cost of overall measurement time delay torespond to a valid condition. In another example, the mobile stationobtains RCPI measurements of the signals, which may be averaged over aninterval, the duration of which may be configurable.

As is known in the art, a mobile station and the base station with whichit is associated or otherwise connected will derive or negotiate oradapt, or any combination thereof, to an actual data rate forcommunications over the link between the mobile station and the basestation. At 504, link quality measurement module 206 may determine anexpected RF power level based on the actual data rate. A look-up tablemay be employed to determine the expected RF power level. The look-uptable may be based on field measured data, laboratory measured data,expected receiver performance, manufacturing calibration data, or anycombination thereof.

An example of such a look-up table for IEEE 802.11b devices follows,assuming a data rate algorithm derives a packet error rate of <1 in 1000and the radio noise floor plus other radio implementation errors equal−95 dBm:

Actual Data Rate Expected Power Level 11 Mbps  −83 dBm or higher 5.5Mbps   −86 dBm to −84 dBm 2 Mbps −89 dBm to −87 dBm 1 Mbps −90 dBm orlower

An example of such a look-up table for IEEE 802.11g/a devices follows,assuming a data rate algorithm derives a packet error rate of <1 in 1000and the radio noise floor plus other radio implementation errors equal−95 dBm:

Actual Data Rate Expected Power Level 54 Mbps −68 dBm or higher 48 Mbps−72 dBm to −69 dBm 36 Mbps −76 dBm to −73 dBm 24 Mbps −80 dBm to −77 dBm18 Mbps −83 dBm to −81 dBm 12 Mbps −85 dBm to −84 dBm  9 Mbps −87 dBm to−86 dBm  6 Mbps −88 dBm or lower

IEEE 802.11n, which is a proposed amendment to the IEEE 802.11 physical(PHY) layer, is expected to have many more data rates than IEEE 802.11bor IEEE 802.11a/g. This increase in the number of data rates means thatthe dBm ranges of expected power level for an actual data rate will besmaller for any one rate.

Cellular technology currently has limited data rate adaptation, but itis contemplated that more data rate choices will be available in futurecellular network standards.

At 506, link quality measurement module 206 may compare the measured RFpower level to the expected RF power level determined at 304.

At 508, link quality measurement module 206 may determine a measure ofthe quality of the link based on the results of the comparison done at304. The measure of the quality of the link may be abstracted to threevalues (high, medium, low). Abstractions to two values and abstractionsto more than three values are also contemplated.

At 510, link quality measurement module 206 may provide the measure ofthe quality of the link to handoff application module 208.

FIG. 6 is a flowchart of an exemplary method for determining a measureof the quality of a link based on the results of a comparison between anexpected RF power level and a measured RF power level. In this exemplarymethod, the quality of the link has been abstracted to three values:high, medium and low.

If the measured RF power level is within x “steps” or x dB of theexpected RF power level on the scale of allowable data rates, as checkedat 602, then the quality of the link is determined at 604 to be “high”.If the measured RF power level is y or more “steps” or is at least y dBabove the expected RF power level on the scale of allowable data rates,as checked at 606, then the quality of the link is determined at 608 tobe “low”. If neither of these conditions is met, then the quality of thelink is determined at 610 to be “medium”. The values of x and y willdepend upon the total number of allowable data rates and on how theabstraction is to be handled either as a finite number of explicit datarates or are converted relative to a common measure such as dBm.

It is obvious to a person of ordinary skill in the art how to modify themethod of FIG. 6 to abstractions of two values and to abstractions ofmore than three values.

Although the subject matter has been described in language specific tostructural features or methodological acts or both, it is to beunderstood that the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A method in a mobile station that is associatedwith a base station, the method comprising: determining a measure of thequality of a link between the mobile station and the base station bycomparing either: i) an expected data rate for communications over thelink and an actual data rate of communications over the link, whereinthe expected data rate takes into account radio frequency power levelsof signals received at the mobile station over the link, or ii) anexpected radio frequency power level for communications over the linkand radio frequency power levels of signals received at the mobilestation over the link, wherein the expected radio frequency power leveltakes into account the actual data rate of communications over the link.2. The method of claim 1, further comprising: determining whether totrigger the mobile station to initiate a handoff from the base stationbased, at least in part, on the measure of the quality of the link. 3.The method of claim 2, further comprising: selecting another basestation as a target of the handoff, on the expectation that a link withthe other base station will experience less interference than that whichis experienced on the link between the mobile station and the basestation with which the mobile station is associated.
 4. The method ofclaim 1, further comprising: determining based, at least in part, on themeasure of the quality of the link whether to transfer to the link acommunication session currently being carried over a different linkbetween the mobile station and a different base station.
 5. The methodof claim 1, further comprising: using signal strength measurements ofthe signals as an indication of the power levels.
 6. The method of claim1, further comprising: using received channel power indicatormeasurements of the signals as an indication of the power levels.
 7. Amobile station comprising: a wireless local area network communicationinterface through which the mobile station is able to communicate withan access point over a communication link; a processor coupled to thewireless local area network communication interface; a memory coupled tothe processor, the memory storing a link quality measurement modulethat, when executed by the processor, is arranged to determine a measureof the quality of the link by comparing either: i) an expected data ratefor communications over the link and an actual data rate ofcommunications over the link, wherein the expected data rate takes intoaccount radio frequency power levels of signals received at the mobilestation over the link via the wireless local area network communicationinterface, or ii) an expected radio frequency power level forcommunications over the link and radio frequency power levels of signalsreceived at the mobile station over the link via the wireless local areanetwork communication interface, wherein the expected radio frequencypower level takes into account the actual data rate of communicationsover the link.
 8. The mobile station of claim 7, wherein the memorystores a handoff application that, when executed by the processor, isarranged to determine whether to initiate a handoff from the accesspoint based, at least in part, on the measure of the quality of thelink, and is arranged to control the wireless local area networkcommunication interface accordingly.
 9. The mobile station of claim 8,wherein the handoff application, when executed by the processor, isarranged to select a base station as a target of the handoff, on theexpectation that a link with the base station will experience lessinterference than that which is experienced on the link between themobile station and the access point.
 10. The mobile station of claim 7,wherein the link quality measurement module, when executed by theprocessor, is arranged to use signal strength measurements of thesignals as an indication of the power levels.
 11. The mobile station ofclaim 7, wherein the link quality measurement module, when executed bythe processor, is arranged to use received channel power indicatormeasurements of the signals as an indication of the power levels.
 12. Amobile station comprising: a wireless local area network communicationinterface through which the mobile station is able communicate with anaccess point; another wireless communication interface through which themobile station is able to communicate with a base station over acommunication link; a processor coupled to the wireless local areanetwork communication interface and to the other wireless communicationinterface; a memory coupled to the processor, the memory storing a linkquality measurement module that, when executed by the processor, isarranged to determine a measure of the quality of a link between themobile station and the base station by comparing either: i) an expecteddata rate for communications over the link and an actual data rate forcommunications over the link, wherein the expected data rate takes intoaccount radio frequency power levels of signals received at the mobilestation over the link via the other wireless communication interface, orii) an expected radio frequency power level for communications over thelink and radio frequency power levels of signals received at the mobilestation over the link via the wireless local area network communicationinterface, wherein the expected radio frequency power level takes intoaccount the actual data rate of communications over the link.
 13. Themobile station of claim 12, wherein the memory stores a handoffapplication module that, when executed by the processor, is arranged todetermine whether to initiate a handoff from the access point based, atleast in part, on the measure of the quality of the link, and isarranged to control the wireless local area network communicationinterface accordingly.
 14. The mobile station of claim 13, wherein thehandoff application module, when executed by the processor, is arrangedto determine based, at least in part, on the measure of the quality ofthe link whether to transfer to the link a communication sessioncurrently being carried over a different link between the mobile stationand the access point.
 15. The mobile station of claim 12, wherein thelink quality measurement module, when executed by the processor, isarranged to use signal strength measurements of the signals as anindication of the power levels.
 16. The mobile station of claim 12,wherein the link quality measurement module, when executed by theprocessor, is arranged to use received channel power indicatormeasurements of the signals as an indication of the power levels.